1. Field of the Invention
This invention relates to electrochemical cells in general and more particularly to an improved air cathode used in metal-air batteries.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Metal-air batteries, such as zinc air batteries offer the advantage of very high energy densities over non-conventional batteries such as alkaline, nickel, lithium ion, cadmium and other high density batteries. Zinc air batteries can be manufactured on a commercial production basis at low cost with a high degree of safety. Rechargeable zinc batteries suitable for use in automotive applications as the primary power source use a liquid electrolyte and often include a pump to recirculate the electrolyte. Such systems are impractical for miniature consumer applications ranging from radios to portable computers because of the mechanical complexity and other problems.
There have been several attempts to build batteries for small or miniature applications, such as illustrated in U.S. Pat. No. 4,957,826 issued to Michael C. Cheiky on Sep. 18, 1990. The Cheiky patent discloses a zinc anode plate completely wrapped in an absorbent cloth anode separator containing liquid electrolyte. A hydrogel material packed in an inert mesh separator is sandwiched between the wrapped anode and an air cathode. The gel material is sufficiently permeable to oxygen, to allow oxygen flow therethrough during charging and discharging cycles of the batteries, and provides chemical reactive communication to the air cathode and the electrolyte in the anode separator without allowing electrolyte to pass therethrough. The permeability of the gel material is selected, such that during charging of the battery, oxygen generated by the anode exerts sufficient pressure at the interface of the anode separator and gel material to cause electrolyte to be recirculated through the edges of the cloth anode separator to the opposite layer thereof, thereby preventing depletion of electrolyte to the anode. The battery of the Cheiky invention uses a metal powder or paste such as zinc in a semi-permeable foam, packed in an electrically conducted mesh screen to form an anode. A more recent metal-air cell similar to that disclosed in U.S. Pat. No. 4,957,826 to Cheiky is disclosed in U.S. Pat. No. 5,306,579 to Shepard Inc., et al. This patent discussed a bifunctional air-electrode having an active layer with an oxygen reduction catalyst and an oxygen evolution catalyst so as to provide a larger number of charge-discharge cycles.
Another U.S. patent by Sammells, et al., U.S. Pat. No. 4,328,287 discloses an air-metal battery cell having a porous electrode through which is passed a colloid of an oxidizable gas and electrolyte. The cell may be a primary cell with a anode-electrode comprising consumable metals, such as zinc. Electrolyte is supplied to a colloid-forming region in the cell by electrolyte supply means, which includes an electrolyte recycle pump means and electrolyte treating means for removing reaction products from the electrolyte and suitably treating the electrolyte for recycling through a distributor and manifold to the colloid-forming region of the cell.
Air-metal cells are well known in the art and generally comprise a consumable metal anode, a catalytic non-consumable oxygen consuming cathode, and a suitable electrolyte. Some of these primary fuel cells are multi-cell batteries with replaceable anodes. Other types of air-metal cells include those which are manufactured in the form of miniature button cells for hearing aids and alike. The typical zinc-air button cell generally includes: a cathode having at least one air port for the entrance of air and which contains a non-consumable air cathode structure usually comprising a gas permeable hydrophobic polymer film onto which is bonded a metal current collection grid and a waterproofed porous catalyst material, such as metal catalyzed active carbon mixed with a hydrophobic binder. The button cell also includes an anode container or can which is joined to the cathode typically by crimping and which includes a zinc anode mass, typically in the form of amalgamated zinc powder, or a porous zinc which is compacted and saturated with alkaline electrolyte, such as 30 to 40% aqua solution of KOH. The battery will also include an insulator between the cathode and anode, typically made of polyethylene, polypropylene, nylon, and the like, which can function as an electrolyte seal. A typical mixture button cell is disclosed in U.S. Pat. No. 5,308,711 issued to Passaniti, et al. The patent also discloses an air cathode.
Air-metal batteries such as zinc-oxygen cells had been used as a power source for electric vehicles and the like, because they provide high energy density relative to other cell chemistries. Zinc-oxygen cells have also been found suitable because they may be recharged by mechanically replacing the zinc electrode, by replacing the liquid electrolyte which contains zinc particles or by electro-mechanically replenishing zinc to the anode while also making available a fresh oxygen supply.
U.S. Pat. No. 4,009,230 teaches an air-zinc batteries having air passages through an active carbon cathode which is surrounded by gelled electrolyte. U.S. Pat. No. 4,137,371 describes a zinc-oxygen cell having a zinc electrode, and an oxygen porous diffusion cathode with a zincate ion diffusion restricting membrane joined directly to the oxygen electrode between the porous layer of this electrode and the zinc electrode. This is for purposes of preventing poisoning of electrochemically active material by zincate ions.
Another U.S. Pat. No. 5,445,901, issued to Korall, et al. on Aug. 29, 1995, discloses a multi-cell sealed zinc-oxygen battery comprising a container containing a plurality of bi-cells, each cell having a housing provided with two major surfaces and accommodating a pair of oppositely disposed space-apart air permeable, liquid impermeable cathodes in the form of oxygen reduction electrodes, and defining between them a cavity configured to accommodate an anode of the battery and electrolyte.
From the above discussion it is seen that air-metal batteries or cells come in many forms and sizes and have been found to provide an effective way to provide portable electrical power to a wide range of devices from electrical automobiles to miniature hearing aids. Of course, of utmost importance to each of these different types of batteries or cells is an effective and efficient air-cathode.
Much of the earlier prior art related to air cathodes used porous carbon or graphite, which, unfortunately is inherently structurally too weak to be used in thickness much below 1/8th inch. Electrodes based on rigid, relatively thick porous carbon plate or blocks, have also been studied extensively with emphasis on the effect of pore diameter, on gas permeability and electrode performance. Beside being bulky, these thick plates are not uniformly porous.
Thin porous carbon paper based electrodes, such as disclosed in U.S. Pat. No. 3,912,538, solves the bulk problem and has a shortened diffusion path. Unfortunately, thin porous carbon paper substrates are very fragile, and they are subject to excessive flooding with electrolyte which interferes with the access of the gas to the electro catalytic sites of the electrodes. To control the flooding, the carbon papers are often rendered hydrophobic by means of, for example, a Teflon coating which increases their electrical resistivity. In addition, because they are structurally weak, they tend to break in handling, as well as when they operate under moderate gas pressures. Finally, the wet-proofed carbon papers have to be dense to provide a minimum of structural integrity. This characteristic confines a catalytic layer to a surface coating bonded merely to one face of the paper substrate, and being paper, they are inherently nonuniform with respect to porosity. Another thin electrolytic gas diffusion electrode comprises a substantially uniform, open pore carbon or graphite substrate, having a thickness in the range to about 5 to 40 mils, and preferably about 10 to about 35 mils and includes a mixture of Teflon or similar wet-proofing particles and catalytic carbon particles imbedded and added within the cloth pores. This type electrode has improved electrochemical performances as well as improve structural strength and is suitable for use in free-flowing electrolytic electrochemical cells. The catalytic carbon particles are either metal-free catalytic carbon particles or finely divided high surface area carbons carrying suitable known noble metal catalytic particles, including platinum, palladium, radium, iridium, ruthenium, and silver, depending on the environment (e.g., acid or alkaline, air or hydrogen, and on operating conditions: temperature, current density and intended length of service).
Suitable substrates are open pore uniform woven cloths made with carbonization at high temperatures of prewoven carbonation fabrics. The carbon content of the cloth should be in excess of 97% by weight and preferably at least 99% by weight to avoid an undesirable impurity interaction with the electrolyte.
Still another U.S. Pat. No. 4,248,682, issued to Linstrum, et al., on Feb. 3, 1991, discloses thin gas diffusion electrodes comprising open pore carbon cloth substrates, provided with the uniform mixture of catalytic carbon particles and preferably Teflon particles adhered within the cloth pores and to the yarns of the cloth, thereby forming an electrode assembly with a plurality of closely spaced, preferably noble metal current collecting contacts. Such a diffusion electrode is typically used in electrochemical cells, including metal-air batteries and zinc electro cells and the like operating at high current densities. According to the U.S. Pat. No. 4,248,682 patent, an electrically conducting thin carbon cloth is coated with catalytic carbon particles mixed in Teflon or other hydrophobic binder, such that they adhere within the cloth openings or pores. Silver current carrying ribbons are interwoven within the cloth to provide current collection throughout the area of the cloth. Although silver is described in the patent, gold and platinum or other noble metals could be used.
U.S. Pat. No. 4,091,175 issued to Hohne of Germany discloses an air electrode for electrochemical cells using silver coated carbon as a catalytic material. The catalytic material disclosed in this patent also contains nickel hydroxide of up to 2% by weight.
A gas diffusion electrode and process is disclosed in U.S. Pat. No. 4,377,496 issued to Solomon. This patent is directed to an oxygen electrode having a conductive porous contacting sintered plaque metal substitute having site depression on the active layer contacting surface. The active layer may use catalyzed carbon particles which interlock with the site depressions. Also included is a hydrophobic backing such as PTFE (polytetrafluoroethylene).
Still another U.S. Pat. No. 4,407,907 issued to Takamura, et al., discloses an air-electrode comprising an electrode body which accelerates the reaction at the microscopic three phase interface of diffused air, the solid electrode body and the electrolyte. The oxygen concentration at the three phase interface is increased by using a fluorine containing solvent in the solid electrode body to improve the water repellant properties of the electrode body.
U.S. Pat. No. 4,927,718 discloses a carbonaceous electrode support material formed by heat treated carbon black material to a temperature above 2500.degree. C. in an inert atmosphere. The support material is then shaped and formed into a catalyst support and then a catalytically active material is added.
U.S. Pat. No. 5,306,579 to Shepard, et al., discussed earlier with respect to batteries discloses a "bifunctional" air-electrode having an oxygen reduction catalyst and an oxygen evolution catalyst so as to increase the number of available charge discharge cycles.
Another U.S. Pat. No. 5,308,711 to Passaniti also discussed earlier discloses an air cathode using manganese compounds as a catalysts which are distributed throughout a carbon matrix. Manganese compounds of valence state +2 form between carbon particles after the carbon particles are added to an aqueous solution of potassium permanganate.